To some extent cetane improvers have been used for many years to increase the cetane number of diesel fuels. Higher cetane value leads to faster engine start especially in cold weather, quieter engine operation, less smoke and possibly less injector coking. Recently use of cetane improvers has greatly increased due to increased demand for diesel fuel and the lower natural cetane number of diesel base stocks caused by more severe refining of crude oil to make unleaded gasoline of acceptable octane number.
Presently the most common cetane improvers are primary alkyl nitrates such as amyl nitrate, hexyl nitrate, octyl nitrate and the like. These can be readily made by reacting a primary alcohol with a mixture of nitric and sulfuric acid. It is known that the cetane increase in most diesel fuels caused by alkoxyalkyl nitrates is much greater on a weight basis than that available from a primary alkyl nitrate. However, alkoxyalkyl nitrates are very difficult to make by the common commercial process of reacting an alcohol with a mixed nitric-sulfuric acid. It is possible to conduct such a nitration of an alkoxy-alcohol as a batch operation at low temperature over a short reaction period. If the reaction period extends over about 5-10 minutes the reaction temperature becomes uncontrollable and violently erupts spewing the entire reaction mixture out of the reaction vessel. This is referred to as a fume-off.
It has been attempted by us to avoid this fume-off problem by conducting the nitration on a continuous basis by feeding the alkoxyalcohol and mixed nitric-sulfuric acid continuously to a reaction vessel and withdrawing reaction mixture such that the average residence time in the reaction vessel is short enough such that the reaction would have been successful as a batch operation. However after a period of time oxidation products begin to build-up in the reaction vessel leading to an increase in decomposition rate and eventually to the same uncontrollable fume-off.
Accordingly, a need exists for a process capable of making alkoxyalkyl nitrates on a continuous basis while avoiding violent and possibly dangerous fume-offs.